A laundry dryer having a heat pump typically comprises a refrigerant circuit and an air path. The refrigerant circuit comprises a compressor, a condenser, a restrictor and an evaporator which are connected in series by refrigerant lines. The refrigerant flows through the compressor, the condenser, the restrictor and the evaporator, in this order. The refrigerant releases heat to the process air flowing through the air path by means of the condenser and extracts heat and humidity from the process air flowing through the air path by means of the evaporator. The compressor absorbs power and compresses the refrigerant in the refrigerant circuit.
Within the air path or process air circuit, process air flows from a drum to the evaporator. At a drum outlet, the air is at a medium temperature and relatively wet. At the evaporator, the air is cooled and dehumidified and then flows to the condenser where it is heated. Hot and dry air is then introduced again in the drum where it can absorb moisture from laundry contained in the drum. The evaporator and the condenser are thus acting as heat exchangers having a refrigerant side and a process air side. The use of a heat pump in clothes dryers and its general layout is well-known in the art, as shown e.g. in the documents EP 2 132 370 B1, EP 2 212 463 B1, U.S. Pat. No. 8,356,423 B2, EP 1 632 736 A2, EP 1 593 770 B1, WO 2013/060626 A1, WO 2013/023958 A1, WO 2012/065916 A1, WO 2011/080045 A1, US 2010/0154248 A1, and US 2011/0209484 A1.
A laundry dryer comprising a heat pump has an improved efficiency in use of energy (e.g., in terms of kWh/kg) as compared to a conventional laundry dryer only employing an electrical heater. Thus, in principle, a related operational carbon dioxide emission of the laundry dryer comprising the heat pump is lower than that of the conventional dryer due to its lower electric consumption. However, a refrigerant used in the heat pump must be taken into account with its GWP (‘Global Warming Potential’). Nowadays, typical refrigerants used in a heat pump are fluorinated hydrocarbon compounds (HFC) like R407C and R134a whose GWP is higher than 1300.
One possibility to reduce TEWI (‘Total Equivalent Warming Impact’, that includes direct and indirect emission) of these systems is to use hydrocarbon refrigerants that have a low GWP like R-290 (propane) and R-1270 (propylene). The main drawback of these refrigerants is that they are flammable and therefore the IEC 60335-2-11 standard limits their maximum charge to 150 g per laundry dryer. It is generally known that an optimum refrigerant charge can be found for a specific system, but the refrigerant limit of 150 g imposed by the IEC 60335-2-11 standard is typically lower than an optimum charge of refrigerant for a conventional heat pump of a laundry dryer.
Efficiency is also affected by the compressor. For example, the efficiency of a rotary compressor is affected by the geometries of its components. The variation of these geometries implies differences in mechanical frictions and in the thermodynamic behaviour of the refrigerant inside the compressor. In more detail, the losses in the compressor that determine its efficiency include the following: energy losses stemming from a motor loss, friction losses, a compression loss due to a not ideal compression, a valve loss due to gas pulsations and an over-compression, and a lubricant pumping loss, as well as mass flow losses stemming from a clearance volume loss due to valve and cylinder head dimensions, a leakage loss, a back-flow-loss, a suction gas heating loss due to a gas density at a cylinder inlet, and a loss due to lubricant flow.
Rotary compressors or “scroll-type” compressors are e.g. described in the documents U.S. Pat. No. 7,029,251 B2, U.S. Pat. No. 6,796,779 B1, U.S. Pat. No. 6,672,852 B1, and U.S. Pat. No. 6,413,060 B1.